theorem Th18:
  p..<*p*> = 1
proof
A1: for i st 1<=i & i<1 holds <*p*>.i <> <*p*>.1;
  dom<*p*> = Seg 1 by FINSEQ_1:38;
  then
A2: 1 in dom<*p*>;
  thus thesis by A2,A1,Th2;
end;
